I think it's quite good on the whole, but I know students who dislike it for various reasons - but also quite a few who do like it. There is a lot that is determined by the school and it's those things that often students complain about - homework not at the right level, or what happens when you don't get a question right etc. It can be set so that you try the question twice, and if you still don't get it, then it tells you to ask your teacher for help, for example. The bookwork codes do seem pointless to me. The schools can determine when different topics are assigned, so another of the complaints - that it asks questions on topics they haven't done yet - is again more to do with how the school is setting it. So is the amount, and whether they have to watch the videos or not - I don't know anyone who watches the videos unless they are stuck! I do find that the videos are almost too specific - it juts shows them how to do that question exactly, rather than being more general about the topic and teaching them the maths behind it.
I think the fact that it asks questions in lots of different ways is helpful, and it often targets some of the common misperceptions and errors when it asks questions - I can see how this might be quite annoying to children, but I think it does work. I think also the fact that it keeps bringing back old questions is very useful. I'd say it's more useful at the younger ages of secondary; I'm not sure the questions are involved enough at the higher end - it teaches the topics ok but not solving a whole problem where you need many skills.
I dont' like that schools use it as the entirety of maths homework, because I think it's not enough on its own. But it has its uses, and some of the things that students complain about are actually really good for them (the mixed topic practice!), while others I'd agree are very annoying (copying out graphs and tables, bookwork codes and checks, etc). And a lot of it is down to how the school uses it, rather than the fault of Sparx itself. Compared to Dr Frost or mymaths or some of the others, though, I think it stands up well - it has more variety in the question types than Dr Frost for example, where you just get 20 questions that are variations on about 3 types.